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The soft body parts of
freshwater bryozoans depicted by
scanning electron microscopy
M.G. WALZL & E.R.Wöss
Abstract: For the first time, special scanning electron microscopical preparation techniques, i.e. chem-
ical dehydration and air drying with hexamethyldisilacane, were used to study the Phylactolaemata.
This approach depicted the growth form of colonies and the outer structures of zooids of Plumatella cas-
miana and P. fungosa three-dimensionally. The structures of cystids and polypids, for example the cystid
wall, lophophore, gut, funiculus and the muscles, are represented using dissected zooids. Moreover, this
technique revealed the structures of both asexual reproduction (the buds, the generation and germina-
tion of statoblasts) and the organs for sexual reproduction (testis, ovary, embryo sac, larva).
Key words: Phylactolaemata, Plumatella, anatomy, sexual propagation, statoblast formation.
Introduction and the boxlike gymnolaemate cheilo-
stomes. In most stenolaemate bryozoans, the
The phylum Bryozoa consists of three
exterior surface is insufficient to differenti-
classes, the Phylactolaemata, Gymnolaema-
ate taxa, and taxonomic separation in that
ta and Stenolaemata (RYLAND 1970). The
group relies heavily on characters visible in
vast majority of bryozoan taxa are marine,
petrographic thin sections (SANDBERG 1977,
inhabiting depths from the intertidal to the
see also Ernst and Scholz et al. this volume).
abyssal. Freshwater bryozoans comprise a
The frontal surfaces of cheilostome zooecia,
smaller number of about 60 species (WOOD
however, are covered by membranes or by
1989) and can mostly be assigned to the
calcified walls which offer varying amounts
class of phylactolaemates. This group exclu-
of morphological details (BANTA 1973; see
sively inhabits freshwater, as do a smaller
also Bader & Schäfer, Novosel, and Vävra
number of gymnolaemate species, all be-
this volume). In the latter case, scanning
longing to the order Ctenostomata. All
electron microscopy (SEM) is broadly used
freshwater bryozoans lack mineralized skele-
to study bryozoan skeletons and has became
tons. This is in striking contrast to most bry-
the standard tool in taxonomic work for
ozoans, which have skeletons made of cal-
both palaeontologists and biologists. Even
cite or, less frequently, aragonite. All Steno-
in ctenostomate bryozoans, where calcified
laemata and the order Cheilostomata of the
skeletons are missing, the process of bioim-
Gymnolaemata possess calcareous skeletons;
muration (the preservation of an organism
they are among the most common groups of
by the skeletal overgrowth of a neighbour-
macrofossils found in the post-Cambrian
marine fossil record (TAYLOR 2005). In to- ing encruster) allows the details of the
tal, 14-700 species have been described in zooids to be described using SEM techniques
the fossil record (HOROWITZ & PACHUT (VOIGT 1966; TAYLOR 1990; TODD 1994).
2000), and about 5.600 extant bryozoans are
In the phylactolaemate group, the soft-
known (TODD 2000). The morphological
bodied zooecia offer less distinctive charac-
features of bryozoan skeletons form the basis
ters (KRAEPLIN 1887, 1892; BRAEM 1890;
for classifications, and these features differ
HYMAN 1959; LACOURT 1968; WOOLLA- Denisia 16, zugleich Kataloge
between the tubular stenolaemate bryozoans der OÖ. Landesmuseen
COTT & ZIMMER 1977; MUKAI 1982). In Neue Serie 28 (2005), 49-58
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contrast to marine species, where hetero- there undisturbed at least until most of the
zooids such as avicularia and vibracularia are polypids had protruded. Then, with a
present (RYLAND & HAYWARD 1977; HAY- pipette, a saturated aqueous solution of
WARD & RYLAND 1979), the uniform shape chloral hydrate (C1CCH(OH)) was added
3 2
of the autozoids of phylactolaemate species dropwise and carefully to the water surface.
has raised less interest for SEM studies. Ul- The specifically heavier chloral hydrate so-
trastructural investigations on spermatozoan lution sinks down to the colony and narco-
structure and larva (FRANZEN 1982, FRAN- tizes the zooids, so that most of the polypi-
ZEN & SENSENBAUGH 1983) sporadically in- des remain protuded. After 15 minutes, an
volve SEM. Most ultrastructural work, how- equal volume of 1 % aqueous buffered
ever, has traditionally focused on stato- formaldehyde solution with pH 7.2 (LlLLIE
blasts. These dormant bodies are excellently 1954) was added. For definitive fixation the
suited for SEM because their shell consists colonies were removed and immersed into a
of a chitinised cuticula. They have been the
4 % buffered formaldehyde solution. The
target of numerous comparative morpholog-
colonies were stored in this medium until
ical investigations (e.g. WlEBACH 1974;
examination.
RAO & BUSHNELL 1979; MUKAI 1999;
For further detailed SEM investigations,
MUNDY 1980; GOETHALS et al. 1984; ODA &
selected parts of colonies or single zooids
MUKAI 1985; GEIMER & MASSARD 1986;
were separated from the substratum using a
POURCHER & D'HONDT 1987; WOOD &
sharp razor blade and transferred to distilled
WOOD 2000). In some cases, such as within
water to wash out excessive formaldehyde.
the genus Plumatella, SEM has become the
The distilled water was changed 3 times af-
only reliable tool for species distinction
ter 15 minutes (important to avoid precipi-
(GEIMER &. MASSARD 1987; WOOD 2001;
tation during the following processes). Af-
TATICCHI &. PlERONI 2005 and see also Tat-
terwards, the samples were dehydrated
icchi et al. this volume).
chemically with acidified 2,2-dimethoxy-
Despite the great benefit of SEM, this propane (DMP) (MULLER &. JACKS 1975).
standard method has rarely been used to For acidification and activation, 1 ml 25 %
study the soft-bodied outer and inner parts HCL was added to 100 ml DMP shortly be-
of the zooids (but see: MUKAI et al. 1997), fore use. For rapid dehydration, 1 part water
probably due to the complexity of the prepa- in the sample vials was mixed with 3 parts
ration techniques required. This SEM study DMP. A rapid cooling of the vials docu-
is an introduction for further ultrastructure mented the endothermic chemical process
work on reproduction in freshwater bry- that yields anhydrous methanol and ace-
ozoans. First results are presented in prepa- tone. After 20 minutes (although an
ration techniques of soft-bodied inner and overnight delay has no negative effect) the
outer parts of the zooids of two species, PIu- solution was replaced twice with water-free
matetta fungosa and P. casmiana. These acetone, for 15 minutes in each case. The
species have already been examined with re- acetone was exchanged with HMDS
spect to the formation of sexual and asexual (1,1,1,3,3,3-hexamethyldisilazane), the
propagules and the reproductive cycle in sample initially being immersed for 30 min
freshwater bryozoans (WÖSS 2002). in a 1:1 mixture of acetone and HMDS fol-
lowed by 30 minutes in pure HMDS, and
then air dried on filter paper under a fume
Material and methods
hood (BRAY et al. 1993; NATION 1983). Af-
Colonies of Plumatella casmiana were ter drying, the samples were transferred in-
collected on 17.5.1992 from a pond at Lax- dividually on aluminiumstubs using a fine
enburg (Lower Austria) and colonies of P. pencil. Single zooids or parts thereof were
fungosa on 7-7. and 3.9.1992 from a backwa- mounted with TEMPFIX-thermo glue
ter of the Danube River at Bad Deutsch Al- (Neubauer Chemikalien company, Ger-
tenburg (Lower Austria). many), and parts of colonies or groups of
zooids were mounted using silver paste. All
The colonies were transported in pond-
samples were then sputter coated with 40
water, along with the logs and twigs on
nm of gold in a Agar B 7340 sputter coater.
which they grew, to the laboratory and left
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Fig. 1: Parts of colonies with many protruded polypides of Plumatella casmiana (a) and P. fungosa (b).
Specimens were examined at 10 to 15 ana zooids are smaller, and the horseshoe- Fig. 2: Zooids with protruded polypides of
keV in a Philips XL20 scanning electron mi- shaped lophophore therefore bears only 25- Plumatella casmiana (a) with rough,
incrusted outer cystid wall and collar
croscope and photographed digitally. In or- 40 tentacles, whereas 40-60 tentacles are
region (co) and P. fungosa (b) with smooth
der to view the internal organs, zooids were present in P. fungosa (Fig. 2). The single outer surface of cystid wall.
opened with two tungsten needles that were
sharpened by repeatedly inserting the nee-
dle tips into an aqueous potassium hydrox-
ide solution under 6 volts of alternating cur-
rent generated by a microscope transformer.
The zooids were opened either in the phase
of washing in water or after air drying.
Results and discussion
The colonies of Plumatella casmiana and
P. fungosa are characterized by a different
growth form. Plumatella casmiana shows an
irregularly "knotty" arrangement of zooids,
which are attached to the substrate in a
sheetlike growth form; colonies of P. fun-
gosa, however, are packed more densely,
with regular fused zooids resulting in an
erect and massive growth form (Fig. 1).
The fully grown monomorphic zooids of Fig. 3: Plumatella fungosa - endocyst body wall with epidermis (ep), circular muscles (cm),
both species differ in size. Plumatella casmi- basement membrane (bm), longitudinal muscles (Im) and ciliated peritoneal cells (pe).
51
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Fig. 4: Plumatella looids consist of two functional units, the
fungosa -
cystid (with its protective cover and the
endocyst body
budding region including the female repro-
wall with pits (pi)
arround the edge ductive organs) and the polypid, which
of the orif icium mainly serves for,nutrition and respiration.
(or) and rows of
epidermal cells All cystids of P. fungosa have a smooth
bearing
outer surface, whereas those of P. casmiana
extensions (ex).
have a rougher outer surface usually incrust-
ed by microparticles and tend to form a keel.
A conspicuous feature is that the anterior
end of the P. casmiana cystid has no incrus-
tations and shows a distinct collar region
(Fig. la, 2a).
The body wall of the cystid consists of an
outer non-living part and an inner living
part, which ends with the outermost epider-
mis. The inner part of the body wall (endo-
cyst) is not connected rigidly with the outer
part of the body wall (ectocyst). Therefore,
during preparation, the endocyst can be eas-
ily pulled out from the chitinuous ectocyst
tube (sheath) to investigate the surface of
the epidermis. The endocyst wall is multi-
layered, and its mesodermal part is separated
from the ectodermal part by a basement
membrane. On the outside of the basement
membrane is the one-layered epidermis, with
a circular muscle layer in between; on the in-
ner side there is the peritoneal epithelium of
the metacoel, underlain by a longitudinal
muscle layer (Fig. 3). The innermost layer of
the endocyst wall is the peritoneum of the
Fig. 5: Plumatella fungosa - yuuny, budded zooid, polypide retracted, regularly
distributed pits (pi). metacoel. In certain areas of this coelotheli-
um, groups of cells bear cilia for circulation
of the coelomic fluid. Each cell has clearly
visible cilia arranged in a row like the teeth
of a comb (Fig. 3). This arrangement proba-
bly more effectively transports coelomic flu-
id. At some specimens, certain ectocyst-
sheath-secreting epidermal cells bear con-
spicuous extensions, probably to anchor the
endocyst to the ectocyst tube. In P. /ungosa
these epidermal extensions are sometimes
arranged in series of rows (Fig. 4). The ori-
fice of the cystid is sometimes surrounded by
pits (Fig. 4), and especially the young :ooids,
Fig. 6: Plumatella casmiana, dorsolateral
view - polypide protruding from the
orificium (or) of the cystid (cy) with the
horseshoe-shaped lophophore (lo) and the
anus (an) outside the lophophore. Ciliated
tentacles (te) originate on the lophophore,
and are interconnected at their base by an
intertentacular membrane (tm).
52
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Fig. 7: Plumatella fungosa - tentacles with
rows of multiciliated cells at their lateral and
inner sides
shortly after evagination, show a regular
arrangement of these pits. These structures
can he interpreted either as sensory pits or a^
vestibular pores (Fig. 5).
The polypide consists of the lophophorr
and the V-shaped gut, which is connects
with the ventral cystid wall by a hollow
peritoneal cord, the funiculus.
The lophophore bears tentacles inter-
connected at their bases by the intertentac-
ular membrane, a fold of the lophophore
(Fig. 6). Three rows of epidermal cells of the
tentacles bear cilia directed laterally and to
the inner side of the lophophore. The syn-
chronized beating of the cilia generates ;i
water current that transports particles to-
ward the mouth (Fig. 7, 8). The mouth is
situated centrally on the lophophore and is
encircled by the tentacles. Dorsally, the
mouth is overhung by a flap, the epistome,
which can close the opening (Fig. 8). As op-
posed to the mouth opening, the anus lies
outside the tentacle circle at the dorsally
open side of the lophophore (Fig. 6).
The gut is divided into different parts -
the pharynx, oesophagus, cardia, caecum and
the intestine - and hangs into the coelomic
cavity of the cystid. A large, paired muscle on
either side of the gut extends across the meta-
coel. On one side, the muscle inserts in the
ventro-lateral cystid wall, and on the other Fig. 8: Plumatella fungosa - horseshoe-shaped lophophore with tentacles; the mouth is
side at the base of the lophophore, i.e. at the located in the bend of the lophophore, overhung dorsally by the epistome (ep).
tentacle sheath, with two smaller bundles al-
so inserting directly on the gut (Fig. 9). The
gut wall consists of the endothelium and two
layers of muscles, an inner circular muscle
layer and an outer longitudinal layer. At the
bulged end of the V-shaped gut, the peri-
toneum of the gut continues to a hollow peri-
toneal cord, the funiculus (Fig. 10).
The funiculus runs across the metacoel
and passes into the peritoneum of the cystid
wall (Fig. 11). Statoblast formation begins
at the insertion area of the border funiculus-
Fig. 9: Plumatella fungosa - two dissected
zooids with opened cystids (cy). Gut (gu) and
retractor muscles (rm), tentacle sheath (ts),
oesophagus (oe) cardia (ca); caecum (cm),
intestine (in).
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Fig. 10: Plumatella fungosa - tip of the V-
shaped gut with funiculus (fu).
cystid wall (Fig. II). During growth, stato-
blasts are transported toward the gut (Fig.
12) and, after having reached their final
si:e, they detach from the funiculus and are
free-floating in the coelomic fluid, moved
by beating of the cilia of peritoneal cells. In
this phase of development the statoblasts
are still surrounded by the funculus epitheli-
um and by the outer epidermal layer of the
statoblast. Every statoblast floating in the
coelomic fluid shows a different level of de-
generation of the funiculus epithelium (Fig.
13). Statoblasts released by the zooid consist
of two valves formed by the outer epidermal
layer. Under favourable conditions, stato-
blasts germinate by opening of the valves at
the equatorial suture (Fig. 14); the inner liv-
ing ectodermal and mesodermal material
then generates the primary zooid (Fig. 15).
The propagation of zooids via budding
takes place at the ventral cystid wall and fol-
lows a strict pattern (JULLIEN 1885; BRAEM
1890; KRAEPLIN 1892; MARCUS 1925; CORI
1941 )• The main bud generated by the adult
:ooid is followed by an adventive bud situat-
ed between main bud and ovary (Fig. 16).
In temperate zones, sexual reproduction
begins in late spring. In P. casmiana, the go-
nad formation is simultaneous (WÖSS
2002). The testis anläge originates at the
upper part of the funiculus, close to the cae-
cum; it then expands to cover the entire fu-
Fig. 11: Plumatella fungosa - funculus (fu) connected to the ventral inner side of the
cystid wall (cw), with the first small buds of statoblasts (sb). niculus, even between the statoblasts (Fig.
17). Spermatogenesis is clearly visible along
the different sections of the funiculus. Dif-
ferent zooids show the diverse stages, start-
ing with a few large spermatogones, fol-
lowed by spermatids and ripe long-tailed
spermatozoa (Fig. 18). The ovary, a peri-
toneal sac of the cystid wall, is positioned
between the budding zone and the orificium
(Fig. 16, 19; see also d'Hondt this volume).
The small ovary contains only a few eggs.
In ripe zooids, above the ovary, the cys-
tid wall elongates as a second peritoneal sac,
the so-called embryo sac (Fig. 16, 19, 20).
Only one fertilized egg is stored in this em-
Fig. 12: Plumatella fungosa - growing
statoblast (sb) bulging out of the funiculus
(fu).
54
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bryo sac. It develops there into the so-called Fig. 13: Statoblasts of Plumatella fungosa (a) and P. casmiana (b) detatched from the
funiculus and rotating within the fluid of the coelomic cavity, covered by the peritoneal
larva. After embryogenesis, the ciliated lar-
layer (pe) of the funiculus and the outer epidermal layer (oe) of the statoblast.
va is released and swims with the aboral pole
oriented forward (Fig. 21).
Future prospects
This paper is a preliminary study in
terms of future work on classical questions
in zooid anatomy and reproduction. Certain
issues, e.g. in the field of sexual propagation,
were already addressed by the earliest bry-
ozoan workers but remain unsolved until to-
day. The planned studies will focus on re-
production biology as well as on the anato-
my of sensory organs.
To investigate details of gametogenesis,
sperm release, fertilization and nidation of
the egg, and embryogenesis, SEM will be
used in combination with series of semithin
sections and transmission electron mi- Fig. 14: Plumatella casmiana - mature statoblast, the two valves of the shell demarcated
croscopy. In asexual reproduction, the docu- by the equatorial suture line (si).
Fig. 15: Plumatella casmianatwo germinating statoblasts Fig. 16: Plumatella fungosa - inner side of ventral endocyst wall with
surrounded by their valves. main bud (mb), adventive bud (ab), ovary (ov) and embryo sac (es).
55
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Fig. 17: Plumatella fungosa - opened
endocyst with caecum (cm), funiculus (fu),
testis (te) and statoblast (sb).
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